1. Field of the Disclosure
The present invention relates generally to power supplies and, more specifically, the present invention relates to a pulse width modulator circuit.
2. Background
Many switching power supplies use pulse width modulation to regulate an output. A pulse width modulator receives a control signal that is related to the value of the output. The pulse width modulator then sets the conduction time of the switch as a fraction of a switching period according to the value of the control signal. The fraction of the switching period that is the conduction time is the duty ratio of the switch.
The change in duty ratio in response to a change in control signal is a measure of the gain of the pulse width modulator. The gain of the modulator is typically well controlled because the gain has a strong influence on the stability and the dynamic response of the system.
Traditional pulse width modulators use a voltage comparator to compare a control voltage to a triangular or sawtooth voltage from an oscillator. The output of the comparator is the pulse width modulated signal. In applications where the control signal is a current instead of a voltage, a resistor converts the current to a voltage for input to the comparator. The value of the resistor is typically well controlled because it is directly proportional to the gain of the modulator. A problem arises when the traditional technique is used in integrated circuits because it is expensive to implement an integrated resistor with a precise value that does not change with temperature and variations in the process of fabrication.